Incandescent light bulbs are well known and commonly utilized in the lighting industry. When the filament inside such a bulb is heated by an electrical current, light is produced in the visible electromagnetic spectrum. As an incandescent bulb matures, however, the material used to form the filament, e.g., tungsten, evaporates. To extend the bulb life, the bulb is thus typically filled with an inert gas, usually nitrogen, argon or krypton, which reduces the rate at which the filament is evaporated.
It is additionally known that the temperature of the filament has a direct bearing upon the life of the bulb and the light efficiency, i.e., the amount of light emitted from the filament (measured in lumens per watts). That is, a higher filament temperature will achieve higher efficacy and whiter light but will reduce the expected bulb life.
The inert gas within the bulb enclosure may optionally be doped with a halogen element, such as iodine, bromine or chlorine to produce a so-called "halogen" light source. The halogen within such bulbs reacts with the tungsten as it evaporates to form a halogen-tungsten gas complex. When this complex comes into contact with the hot filament, it disassociates back into halogen and tungsten and the tungsten is thereafter redeposited onto the tungsten filament, thereby extending the life of the light bulb.
Halogen bulbs possess a further advantage in that they radiate whiter light, i.e., than that produced by incandescent bulbs, with a higher lumen output due to the higher operating temperature of the filament. However, this elevated level of thermal energy can cause damage both to the glass enclosing the filament and to the ceramic base of the bulb, as well as to the electrical circuitry connecting the bulb socket to a power source.
It is additionally known in the art that only about 4-6% of the electricity passing through the filament of a light bulb is converted into visible light, while greater than 90% thereof is converted into heat (with the minimal remainder being converted to ultraviolet light). For example, in the case of a non-halogen, i.e., incandescent, light bulb operating in a "base down", i.e., facing downward (as this term is used in the art) position, the better than 90% heat conversion rate can cause the glass temperature of a 100 watt general service bulb to reach 450.degree. F. (230.degree. C.). In a base up, or upward facing position, the glass temperature of such bulbs reaches only about 225.degree. F. (108.degree. C.). Thus, a bulb in a base down position retains much more heat than a bulb in a base up position, particularly when the bulb is contained within a fixture, since the heat generated by the bulb becomes trapped within the fixture and can thus significantly increase the glass temperature.
Alternatively, in the case of, e.g., a 60 watt halogen bulb, the glass temperature in a base down position can reach 430.degree. F. (220.degree. C.), i.e., almost that of a 100 watt incandescent bulb, when the bulb is contained in a fixture, even though the wattage of the halogen bulb is only slightly more than 50% of that of such an incandescent bulb. Thus, as would be apparent to one of ordinary skill in the art, halogen bulbs rated for even higher wattages, particularly when enclosed within a fixture, would reach temperature levels substantially above those achieved by incandescent bulbs.
Such elevated temperatures pose a substantial risk of damage, however. For example, excessive heat may lead to failure of the cement affixing the glass to the base. Further, a spray of relatively cool water, e.g., from a lawn sprinkler or during a rain storm, may lead to the formation of thermal cracks in the hot glass. In addition, the elevated level of thermal energy thus produced has also been known to damage the circuitry connecting the bulb socket to a source of electrical power.
A need for an improved lighting fixture capable of overcoming the difficulties encountered with the use of prior art fixtures as described above, which may be used in both "base up" and "base down" orientations, with all kinds of illumination sources has thus long been recognized within the lighting industry. This consideration takes on even greater importance when certain sources, such as halogen bulbs which produce a substantial amount of thermal energy, are mounted in surroundings which lead to the build-up of elevated levels of thermal energy, e.g., within an enclosure such as a fixture.